1. Field of the Invention
The present invention relates to aerobic sewage treatment devices and, in particular, to a type of aeration device that cultivates select species of bacteria beneficial in the digestion of organic and other components of human or biological waste in water.
2. Description of the Related Art
As human population has increased over the past several centuries the capacity of natural digestive processes to deal with human generated organic waste has been exceeded by the sheer volume of such waste.
The art of wastewater treatment has been developed to confront this issue and over the past century substantial advances have increased the capacity for bacterial digestion to process these materials at a higher rate.
Organic wastes are complex but largely consist of carbon-based compounds such as proteins, carbohydrates and fats. The groups of bacteria that process such organic carbon compounds are predominantly those that are referred to as “heterotrophic” bacteria.
Heterotrophic is derived from the root “consumption of others” and reflects the fact that such organisms must derive their nutrition from other pre-existing carbonaceous organic materials, be they of animal or vegetable origin.
Heterotrophic organisms utilize carbon in two fundamental ways, one is for biosynthesis of the proteins and other structural components of cells, the second is to oxidize carbon as a source of metabolic energy.
A second group of bacteria exists that is referred to as “autotrophic”. These organisms are capable of existing without the need to obtain nutrients from pre-existing organic material and are likely to have been the original forms of life on earth.
Autotrophic organisms derive their metabolic energy from the oxidation of compounds other than carbon, for instance reduced nitrogen as ammonia or reduced sulfur as hydrogen sulfide. They still need carbon for biosynthesis of cellular structures but this they obtain from non-organic or mineral sources such as carbonate or carbon dioxide.
While constituents of organic waste are predominantly carbon, there are other compounds that are released by organic digestion that need to be dealt with. The most prominent of these are compounds of nitrogen, a component of protein and urea produced by living cells.
The heterotrophic digestion of organic wastes results in a conversion of organic nitrogen into ammonia, a toxic compound, especially to aquatic life which is exposed when such wastes are released to natural bodies of water as sewage.
It is widely known that wastewater systems that actively aerate, to provide oxygen to heterotrophic bacteria for the digestion of organic carbon, will also eventually be colonized by autotrophic bacteria capable of oxidizing ammonia to NO2 (nitrite). These autotrophs are typically found in the genera Nitrosomonas, Nitrosococcus and others.
Nitrite is also relatively toxic and as the waste product, for example of Nitrosomonas can, if built up in concentration, retard further oxidation of ammonia by Nitrosomonas through the process of “end product inhibition”.
Typically a second group of bacteria will develop in such systems that obtain their energy by oxidizing NO2 (nitrite) to NO3 (nitrate). Genera capable of this include Nitrobacter and Nitrospira. 
Because the conversion of nitrite to nitrate in effect removes the waste product of Nitrosomonas it releases that organism from end product inhibition by nitrite and Nitrosomonas is then free to continue oxidizing more ammonia.
Genera such as Nitrosomonas and Nitrobacter are mutualistic since the first provides the food for the second while the second removes the waste from the first, to their mutual benefit. Such colonies become tightly linked into what is referred to as a “syntrophic” association.
The final product of the above association is nitrate, a stable compound that, while less directly toxic than ammonia or nitrite, has been identified as a serious environmental pollution problem.
Practitioners in the art of wastewater treatment discovered that a biological method for eliminating nitrate exists that is referred to as “bacterial denitrification”. It was discovered that there are heterotrophic aerobic bacteria, known as facultative bacteria, that can survive in anaerobic environments by obtaining oxygen for their metabolism from previously oxidized mineral compounds such as nitrate, sulfate or nitrite.
Nearly all aerobic wastewater treatment technologies developed to date tend to carry the oxidation of ammonia fully to the compound nitrate. Nitrite, the intermediate oxidized form of nitrogen is typically found is extremely low concentrations and is a highly transient component in these systems.
Nitrate is utilized as an oxygen source by facultative heterotrophs only when no source of free oxygen is available. Facultative bacteria process nitrate by first actively transporting the compound across the cell membrane into the cytoplasm and then moving it to the interior of the cell membrane where a membrane bound enzyme strips one oxygen atom from the NO3 to produce the compound NO2 (nitrite). In the process the bacteria obtains a single oxygen atom for use in oxidative metabolism. The NO2 must then be actively transported across the cell membrane back to the outside of the cell, since it is toxic to the organism.
The yield of usable oxygen for oxidative metabolism from nitrate is low, just a single atom, yet the metabolic cost of the transport into and out of the cell is high, so facultative bacteria will use NO3 as an oxygen source only if no free oxygen is available.
Further denitrification from NO2 to N2 gas is a more complex process. The nitrite produced inside the cell of a facultative bacteria, or available in the external milieu, will be processed outside the cell in the periplasm by a series of external membrane bound enzymes that work as a cascade such that NO2 is reduced to NO then combined in a further reduction to N2O than again to the final N2. At each step oxygen is taken into the cell for oxidative metabolism.
The oxygen yield for processing NO2 is two atoms of oxygen for each nitrite molecule. And, since the process does not involve active transport into the cell, the energetics of the reaction from NO2 is substantially different than the reaction starting with NO3.
It can be seen from the above that even in the presence of free atmospheric oxygen nitrite is an attractive oxygen source. Not only is the yield the same as with O2, namely two atoms of oxygen, atmospheric oxygen as O2 is bound by an energetic double covalent bond. Thus a facultative bacteria can obtain oxygen for oxidative metabolism from nitrite without the need to break and energetic double bond, as it would with O2.
U.S. Pat. No. 6,780,318, which is incorporated herein by reference, describes a wastewater treatment device that grows a dense culture of facultative heterotrophic bacteria in wastewater effluent for the purpose of transporting these cultures out to leach fields that have been clogged with anaerobic slime. The facultative organisms eliminate this slime through the process of fermentation. The intent of the device originally was to open the soil so that clogged leach fields could once again percolate effluent into the surrounding soil for treatment.
In this patent a process is described in which the presence of the aggressive facultative heterotrophs prevents the expected invasion of this aerobic system by autotrophic ammonia oxidizers, thus the nitrogen in the system remains unoxidized as ammonia rather than as the more typical oxidized nitrate found in most aerobic treatment systems.
This patent also describes a process whereby the ammonia discharged from such a system into a previously existing leach field will encounter a pre-existing dense colony of ammonia oxidizing autotrophs that formed during the earlier usage of the leach field. When this happens the effluent is accompanied by a dense colony of facultative heterotrophs, grown in the septic tank. As the pre-existing ammonia oxidizers, such as Nitrosomonas or Nitrosococcus begin to form NO2, the facultative heterotrophs immediately begin the denitrification from NO2 as described above. In the process the NO2 will be converted to N2 gas, even if atmospheric oxygen is present.
Nitrobacter or Nitrospira, which would typically oxidize that same NO2 to NO3 are now faced with a competitive organism in the form of the facultative heterotroph, that is considerably more aggressive. Facultative organisms such as Bacillus have a doubling time of as little as 30 minutes while Nitrobacter will double in anywhere from 10-30 hours. As the effluent passes through this zone in the soil the ammonia is directly converted to N2 gas without ever becoming fully oxidized to NO3, thus the reaction can occur even if atmospheric oxygen is present.
U.S. patent application 20050077237 A1 (by Wickham, Daniel, Apr. 14, 2005) for a “Method for recovering a disposal trench with a biomat slime, and method for operating a waste treatment vessel” describes a process whereby the same reaction can be created within the treatment vessel itself by first establishing a colony of autotrophic ammonia oxidizers in the device before the facultative culture is introduced into the system.
When the facultative organisms are introduced they will out compete the Nitrobacter type organisms and the denitrification will proceed as in the soil. With the traditional method, in which ammonia is oxidized to NO3 and then subjected to an anaerobic environment for dentrification, it has proven difficult to sustainably reduce nitrate concentrations to less than 10 mg/l, the federal drinking water standard for nitrate.
Given the need for a technology that can consistently reduce nitrates below the drinking water standard, and the fact that conventional technology consumes a tremendous amount of energy to oxidize ammonia completely to nitrate prior to the denitrification step, a means to sustain the direct aerobic denitrification described above would be of inestimable value to the industry and to society.
Accordingly, it is therefore a first object of the invention to provide a method of wastewater treatment that does not depend on the action of bacteria in a soil leach field or engineered mound septic system to remove ammonia.
It is another object of the invention to provide an apparatus for wastewater treatment that supports the above methods and can be implemented in existing septic tanks of various sizes or modified for use in larger scale wastewater treatment plants and facilities.